Radio frames (also called packets) in a buffer in a mobile phone are transferred from the mobile terminal to the target Access Point (AP), called more often a Node B in 3G, an Access Point in Wireless LAN, and in 2G more often called the Base Transceiver Station. The buffer is maintained by a radio (protocol) layer. It holds all unacknowledged radio frames, whether transmitted or not. When the target AP signals an ACK for a radio frame, the radio layer relays the ACK to an upper layer of the radio protocol, and removes the radio frame from the buffer.
In many radio communication systems of today, a radio layer uses an automatic repeat request (ARQ) method to ensure that transmitted radio frames have been received by the receiving end, i.e. if a radio frame is not acknowledged it is automatically (without a repeat request having been made) re-transmitted. The radio layer therefore maintains the above-mentioned buffer for holding radio frames until they are acknowledged.
Higher data rates envisioned in future wireless technologies impose heavier buffering requirements on the radio layer. In addition, radio frames are radio technology specific (in a multiple radio technology environment), and may be adapted to best suit current radio link conditions, which potentially makes the radio frames in the radio layer between the source and target AP incompatible. Therefore, it would be beneficial to have the buffer maintained not in the radio layer, but in some upper protocol layer, where the data units being transmitted can (and typically do) exist as radio technology independent packets. Further motivating such a change is the observation that upper layer handover occurs less often than radio layer handover, so that it would be necessary to transfer the buffer less often if maintained by an upper layer.
Cellular/wireless handover procedures cause a break in communication sometimes leading to loss of data. In order to minimize or prevent data losses in providing communication for a mobile during a handover, an operator network must know the most recent data that was successfully transmitted to the mobile so that any lost packets can be retransmitted. Although as described above it is beneficial to have buffering at an upper layer, on the other hand to ensure lossless handover one would need an ACK-based scheme between the radio layer and the upper layer. The upper layer would buffer a packet and remove it only when it received a local ACK (an ACK from the radio layer to the upper layer). But the use of a tight local ACK procedure (i.e. one in which every packet must be ACK-ed) would add significant overload to the network and would lead to inefficiency, especially at the higher data rates envisioned in the future.
The invention introduces the use of what is here called a “loose” ACK scheme as a way to provide a lossless handover procedure. In contrast to the prior art—current radio technology uses radio layer buffering (after segmentation and concatenation), e.g. as explained in 3GPP TS 25.322—the loose ACK scheme provided by the invention does not require a tight local ACK procedure (in which the radio layer signals ACKs to the upper layer for each successfully transmitted packet). The loose ACK scheme of the invention is also of use in support of lossless packet delivery during poor radio conditions.